Monomer self-aggregation

The presence of a critical St content in ASt-x can also be seen in fluorescence spectra [29], This copolymer in aqueous solution shows an excimer emission peaking at 325 nra. As shown in Fig. 8, the intensity of the excimer emission increases, while the monomer emission decreases, with increasing St content. Eventually the excimer dominates the monomer emission at an St content of 72 mol%. The excimer emission becomes apparent at an St content of about 50 mol%, which agrees with the critical St content estimated by viscometry and NMR spectroscopy. The existence of the critical St content suggests the hydro-phobic self-aggregation to be a cooperative process. 

Fluorescence quantum yield and emission maximum determinations as a function of peptide concentration may also permit the detection of peptide self-aggregation at concentrations below 10-4 M, because the peptide fluorophore is likely to be located in a different environment in the peptide aggregate. For example, the concentration-dependent changes in the tryptophan fluorescence emission maximum of mellitin were monitored to determine the equilibrium dissociation constant and thermodynamic parameters of the monomer-tetramer self-association reaction of this peptide. 25 Similarly, measurement of the changes in the tryptophan fluorescence intensity of gramicidin A as a function of its concentration permitted the determination of an average monomer-dimer equilibrium con-stant. 26 ... 

Figure 5.12 Chiral self-aggregation of porphyrin compounds (adapted from Ribo et al., 2001, with kind permission), (a) The monomer and J-aggregates structures. (b). The outcome of rotating directions of the flask in the rotary evaporator, clockwise (CW) and anticlockwise (ACW), on the preparation of aggregates by concentration of a monomeric solution of the porphyrin. The corresponding CD spectra, showing the chirality signature, and the UV absorption bands of the J-aggregates are also shown. Notice that the two UV spectra are identical, whereas the two CD spectra are opposite to each other.

The partitioning of alcohol into the oil, water, and interfacial layer domains of a microemulsion controls whether a two-phase or a three-phase microemulsion system is formed, as well as the microscopic characteristics of the microemulsion phases. F or the typical alcohols used, the amount of alcohol present in the oil domain can be large and comparable to the amount present in the interfacial layer. This is in contrast to the behavior of the surfactant, most of which remains at the interfacial layer and only a negligibly small amount of which are partitioned into the oil and the water domains. Therefore, the accurate accounting of the partitioning of alcohol into the oil domain is a necessary part of any quantitative theory of microemulsions. Such a theory must account for the facts that the alcohol is present in the oil phase as both monomers and aggregates and that the self-association of alcohol in the oil is responsible for its appreciable presence in the oil domain. 

All the reported experimental data strongly support that, for the investigated systems, stirring shifts the equilibrium of a racemic mixture towards the side chosen by the vortex chirality. However, various questions remain open. For example, it is not clear at the moment if, under the effect of a vortex, there is chiral enrichment (monomers caught from the eddy sense and pushed to self-aggregate) and/or a racemate resolution, and it is worthwhile to test other similar chemical systems to discover if this simple scheme is more general. 

It will be noted that K-casein is the only monomer subunit that contains a disulfide group, and this is undoubtedly responsible for its ability to self aggregate as well as to interact with g-lactoglobulin during heat processing of milk. 

However, one of the most intriguing examples in this field, which also demonstrates the power of diffusion NMR to characterize supramolecular systems obtained by self aggregation, is the assignment of the different species that prevail in solution following the reduction of 2,5,8,ll-tetra-tert-butylcycloocta[l,2,3,4-def-,5,6,7,8]bis-biphenylene (65) to its respective tetraanions [59]. In this sample, different species were observed and only diffusion NMR provided a proof, in conjunction with 2-D NOESY, that the different molecular species are indeed different helically-stacked anionic aggregates of 65. When NMR diffusion measurements were performed on the obtained solution, four different diffusion coefficients were found for the mixture. These coefficients that were assigned to the monomer, dimer, trimer and tetramer of 65 (Fig. 6.24). Based on Eq. (6.16) ... 

Bile salts carry extensive hydrophobic (hydrocarbon) portions in each molecule that attempt to reduce their contact with water (4). This is reflected in rapid, dynamic association-dissociation equilibrium to form self-aggregates or micelles as the total concentration of bile salt solute is increased (the CMC) [2-6]. Experimentally, micelles are undetectable in dilute solutions of the monomers, and are detected in increasing numbers and often size above the CMC [98]. Because bile salt micelles are often small (i.e., dimers) [5], and since self-aggregation continues to proceed in many cases with increasing concentration above the CMC [17,18,20,52,98], the detection of the lowest concentration at which the first aggregates form depends particularly upon the sensitivity of the experimental probes employed [98] and the physical-chemical conditions [3-5]. 

Fig. 2 Schematic diagram of a monomer isomers studied b monomer self-assembly equilibria and fibrous aggregate polymerization scheme with horseradish peroxidase. Raman scattering intensities as a function of unpolymerized monomer concentration are presented for c DEDT at pH 6.0, and d DELT at pH 6.0. SEM images are presented for the enzymatically polymerized e DEDT at pH 6.0 Scale bar = 2 jxm), and f DELT at pH 6.0 Scale bar = 2 xm). This figure was reprinted with permission from [68]...

Significant solvent-dependent spectral variations may also arise from self-aggregation of dyes in solution. The monomer may be solvatochromic. and it may not be easy to distinguish the contribution of self-aggregation to the observed solvatochromism, especially in media where the dye is poorly soluble. 

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